Once you are lean, move up in Fitness

You need to keep your energy (food) intake down, and exercise at low intensities nearly every day to get that body of yours trim.

Low-intensity aerobic exercise will burn up body fat, so once you are lean, you can keep your body-fat levels down by low-intensity aerobic exercise every second day.

You can use the time you have left over to step up to the next level of fitness, where you train your body to absorb a higher level of physical stress. These are the kinds of stresses you feel when you get involved in sports, or take up running, use weights or resistance training, or attend intermediate-level aerobics classes.

This form of exercise is called high-intensity aerobic training (or HIA training in the jargon we use - can you guess what we call low-intensity aerobic training?). You increase the intensity of exercise or sport by increasing the amount of work you do in the same time.

Examples of this would be to run faster, lift heavier weights, or move more quickly in an aerobics class. High-intensity aerobic training will not significantly decrease your body-fat levels. It will give you the ability to exercise continuously at higher levels, with minimum fatigue.

High-intensity aerobic training will also help you develop more muscle shape, or muscle definition. This kind of training needs to be done twice or three times per week.

It makes a useful addition to your low-level aerobic (LIA - did you guess right?) training. The main problem with high-intensity aerobic training is that you start to use body sugars (glycogen) as the main fuel.

At higher exercise intensities, your body finds it easier to get sugars (rather than fat) to the working muscles, and to break them down for energy supply once they are there. Using sugars as a fuel is not necessarily bad, but the stores of sugars your body can use as fuel are limited.

Once body-sugar levels get low you start to get that tired, drained feeling, and feel as though you have to slow down. You may also start to get dizzy. Your brain relies on sugars as a fuel, and will not tolerate low blood-sugar levels.

Dizziness is a message to your body to stop the activity, lie down, and let your brain soak up more sugars. Learn to recognise and avoid this lethargic feeling. Slow down your activity before you get to the point of fatigue.

The best way of working out how hard you should be exercising for high-intensity aerobic training is to take your pulse, as you did last week.

For high-intensity aerobic exercise, your heart rate should be between 25-30 beats per 10 seconds if you are younger than 20, 23-28 beats per 10 seconds if you are younger than 30, 21-26 beats per 10 seconds if you are younger than 40, 19-24 beats per 10 seconds if you are younger than 50, 17-22 beats per 10 seconds if you are younger than 60, and 15-20 beats per 10 seconds if you are younger than 70.

The kind of training you do for high-intensity aerobic conditioning is called interval training. This means that you alternate an interval of high-intensity work with an interval of low-intensity work.

You push your heart rate up towards the top of the training range in the high-intensity intervals, and let it come down to the bottom of the range in the low-intensity intervals.

This is the way that most intermediate level aerobics classes are designed: you do a track of hard work, then a track of easier exercises.

If you are walking or running, warm up first, then go slightly faster for three to five minutes, slow down for three to five minutes, and repeat this six to ten minute "interval" a couple of times before going into recovery.

In a gym circuit, your instructor should design the exercise stations so that there is a regular "rest stop", such as an exercise bike, at every third to fifth station.

When you get to a rest stop, take your pulse. If it is up towards the top of your high-intensity training range, use low-intensity exercise at the rest stop to get it back down to the bottom of the range.

Make sure you have at least 12 weeks of regular low-level aerobic training before starting high-intensity aerobic training. Do not overdo high-intensity training or you will be training your body to become fatigued, not fit.